1. Field of the Invention
The present invention relates to a heating unit configured to heat a process object placed on the heating unit.
2. Description of the Related Art
A semiconductor device manufacturing apparatus or the like with which a chemical vapor deposition (CVD) step, a dry-etching step, and the like are performed, uses a heating unit configured to heat a substrate serving as an object to be processed, and being placed on the heating unit. The heating unit includes a resistance heating element as a heater. The resistance heating element is buried in a placement surface which is almost parallel to a surface of a base made of an insulating material such as a ceramic. In general, a coil-shaped heating wire and a flat-shaped heating wire such as a mesh, a screen-printed body or foil are used as a resistance heating element (see Japanese Patent Application Publication No. Hei 11-204238).
Hereinafter, descriptions will be given, for example, of a coil-shaped heating wire used as a resistance heating element. The coil-shaped heating wire has multiple corner portions each curved with a certain curvature. This structure makes it difficult to arrange the coil-shaped heating wires side by side without leaving any space therebetween in the placement surface almost parallel to the surface of the base made of an insulating material. As a result, temperature distribution corresponding to the coil shape (referred to as a “wiring pattern”) of the coil-shaped heating wires is caused in the surface of the base. Depending on the temperature distribution thus caused, the thickness of a film deposited on the substrate and the etching depth vary. This variation reduces the yields.
A shifted arrangement of two coil-shaped heating wires has been proposed as measures to solve the problem of temperature distribution which occurs in the surface of the base, and which corresponds to the wiring pattern of the coil-shaped heating wire (see Japanese Patent Translation Publication No. 2006-500789). Nevertheless, it is disclosed that the coil-shaped heating wires cause local temperature distribution with a variation of approximately several degrees Celsius. In other words, the temperature distribution corresponding to the wiring pattern cannot be eliminated by the proposed measures fully.
In a case where a flat-shaped heating wire such as a mesh (sheet formed by weaving metallic wires into a wire sheet shape), a screen-printed body or foil is used as a resistance heating element, ingenious arrangement of the wiring pattern of the flat-shaped heating wire makes it possible to reduce an area which is not directly heated by the heating wire in the surface of the base. When, however, multiple flat-shaped heating wires are placed in the same surface, areas which are put aside for securing an insulation distance between the flat-shaped heating wires need to be set up. The area set up for securing the insulation distance is not directly heated. In other words, the temperature distribution corresponding to the wiring pattern cannot be eliminated fully by such a flat-shaped heating wire used as the resistance heating element.
Another proposal is made on a scheme of stacking multiple ceramic green sheets as follows. Heating elements are formed in the upper surfaces of the respective ceramic green sheets. The heating elements are shifted from one another when viewed as a plan view from the top surface of the substrate on which the wiring patterns of the heating elements are placed (see Japanese Patent Application Publication No. 2002-373862). Even with this scheme, in contrary, it is actually difficult to even out the temperature throughout the surface of the bases, because the wiring patterns of the respective heating elements do not overlap one another in some locations when viewed in the same plane.